1. Field of the Invention
The present invention relates to an illumination optical system that is used for image projection apparatuses such as liquid crystal projectors.
2. Description of the Related Art
Image projection apparatuses have an illumination optical system to introduce light emitted from a light source to an image forming element such as a liquid crystal panel so as to illuminate the image forming element with the light, and have a projection optical system to project the light that is enlarged and to which image information is added by the image forming element onto a projection surface such as a screen.
Many of such image projection apparatuses use as the light source a discharge arc tube such as a high-pressure mercury lamp or a metal halide lamp. The discharge arc tube that a high voltage is applied between its paired discharge electrodes away from and facing each other generates an arc therebetween to emit light therefrom.
On the other hand, many of the illumination optical systems use, in order to evenly illuminate the image forming element with the light from the light source, as disclosed in Japanese Patent Laid-Open No. 2003-186111, a fly-eye lens array constituted by a plurality of lens cells two-dimensionally arrayed.
FIG. 16 shows the illumination optical system including such a fly-eye lens array (hereinafter simply referred to as “a fly-eye lens”), which is disclosed in Japanese Patent Laid-Open No. 2003-186111. Light from a light source 21 constituted by a discharge arc tube and a reflector is divided into a plurality of light fluxes by a plurality of lens cells of a first fly-eye lens 22, and the divided light fluxes are condensed by the lens cells to form arc images as light source images near a second fly-eye lens 23. A polarization conversion element 25 is disposed immediately behind the second fly-eye lens 23.
The polarization conversion element 25 is produced by arraying, with a constant pitch, a plurality of polarization beam splitters each having a width (height) corresponding to ½ of an array pitch of the lens cells constituting the second fly-eye lens 23 and by disposing half-wave plates on alternate ones of entrance surfaces of the polarization beam splitters. The light emitted from the light source 21 which is non-polarized light is converted by a polarization conversion effect of the polarization conversion element 25 into linearly-polarized light having a polarization direction in a specific direction. However, of the light emitted from the light source 21, only partial light entering the polarization conversion element 25 through its effective entrance areas receives the polarization conversion effect.
As shown as hatched areas in FIG. 17 showing the polarization conversion element 25 viewed from a light source side, light blocking members 24 are disposed in non-effective entrance areas of the polarization conversion element 25. Light not entering the polarization conversion element 25 through its effective entrance areas by being blocked by the light blocking members 24 becomes loss light. On the other hand, light forming part of the arc image AI and not entering the lens cell of the second fly-eye lens 23 provided in a one-to-one relation with the lens cell of the first fly-eye lens 22 also becomes loss light.
That is, of the light passing through the first fly-eye lens 22 and forming the arc image AI, light other than light entering the polarization conversion element 25 through the effective entrance areas and further entering the lens cells of the second fly-eye lens 23 becomes loss light, which deteriorates utilization efficiency of the light from the light source.
In the illumination optical system disclosed in Japanese Patent Laid-Open No. 2003-186111, the light source 21 is constituted by the discharge arc tube whose arc direction (direction in which the discharge electrodes are arranged) is set parallel to an optical axis of the illumination optical system and the reflector having a paraboloidal shape rotationally symmetric about the optical axis as an axis of symmetry.
In such a configuration, the arc images AI are formed radially about the optical axis as shown in FIG. 17 and have mutually different orientations. In this case, it is difficult to effectively cause all the light forming the arc images Al to enter the polarization conversion element 25 through the effective entrance areas and further enter the lens cells of the second fly-eye lens 23.
Moreover, discharge arc tubes such as high-pressure mercury lamps have a characteristic that tips of the discharge electrodes are worn out with increase of an accumulated light emission time and a length of the arc is thereby increased. The increase of the arc length increases sizes of the arc images, which decreases the light entering the polarization conversion element 25 through the effective entrance areas and further entering the lens cells of the second fly-eye lens 23. Thereby, the utilization efficiency of the light from the light source is significantly deteriorated. Thus, the conventional illumination optical system using the discharge arc tube is sensitive to the increase of the arc length, and its luminance is likely to be decreased. In other words, the wear of the discharge electrodes directly causes the deterioration of the utilization efficiency of the light from the light source.